Permaculture Designers Manual
CHAPTER 5 – CLIMATIC FACTORS IN PERMACULTURE
Section 5.8 –
Latitude Effects in Permaculture
Despite the weak light and short growing season at high (sub-polar) latitudes, the very long summer days provide more than a sufficient quantity of light for vigorous plant growth.
The daily total for late summer (July) is 440 Langley’s at Madras, India (13°N); 680 Langley’s at Fresno, California (36°N); 450 at Fairbanks, Alaska (64°N).
The average radiation in temperate areas is 1-5 times that of the tropics (Chang, 1968).
This is accentuated, on or near coasts, by the moderate temperatures from the convection of air over warm currents in such areas as Alaska and the northwest coastal regions of Europe.
The benefit of these areas is that the generally lower temperatures, which suit photosynthesis, confer a photosynthetic efficiency that makes a considerable production of cereal, berries, tomatoes, potatoes, and vegetable crops tolerant of short season/long day conditions.
The often deep peri-glacial soils provide the basis for the production of gigantic lettuce, cabbage, spinaches, and root crops, so that these areas are very favorable for agriculture in summer.
Such conditions prevail in Alaska, Ireland, Scotland, and parts of Norway.
Shelter and added nutrients from seaweeds and manures yield rich meadows and heavy vegetable production during the long summer days.
The small stone walled fields of Ireland produce abundant sweet hay, root crops, and greens for storage during winter.
Conversely, the ample light at low (equatorial) latitudes is inefficient due to the extremely high temperatures there, and the excess light may mean that plants are light saturated.
Photosynthesis may actually decline in the intense light and the energy built into the plants may be less in sunlight than in partial shade.
Shade (down to a level of 20% sunlight) is of great benefit in tropical deserts and sunny equatorial climates.
Trials of shade cloth with 50-70% light transmission may greatly increase plant bulk and production, e.g. of sugar beet, thus the importance of tree shade and shade cloth in deserts and cleared-area tropics.
Similarly, temperatures above 25°C (77°F) sharply decrease photosynthetic efficiency, so that the normal desert or equatorial condition of high light and temperature is very inefficient for the production of plant material.
In the arctic or high latitudes, 15°C (59″F) is optimum for adapted species and cultivars and 20-24°C (68-75°F) for many useful food plants.
Tropics are noted for a low production of those crops which can be also grown in temperate areas; light shade may be the essential component for increased yields.
In bright sunlight, leaf temperatures often exceed air temperatures, so that the diffuse light of overcast or cloudy days in high latitudes helps plant growth, especially after midday as temperatures would then also rise above optimum in direct sunlight.
Photosynthetic efficiency is limited by the ability of the leaf to obtain carbon dioxide or by low levels of available carbon dioxide.
At high light intensity, we need to supply carbon dioxide (to the saturation level of 0.13% to obtain a 2-3 times increase in photosynthetic rate.
Carbon dioxide can be supplied by composting or by housing animals in greenhouses where light is more than sufficient.
It follows that the summer periods of the high latitudes are ideal for biomass production, while equatorial regions evolve biomass mainly as a result of a year-round (inefficient) growth and perennial crops.
The ideal of steady low light / low temperature conditions may be at times achieved below the closed forests of tropical mountains, but these sites are very limited in extent, and carbon dioxide concentration is also low.
Rice, for example, yields 4-5 times better in temperate areas than in tropical ones, although up to three crops per year in the tropics helps to increase local yields over the year.
It should be feasible to assist tropical crop yields by spacing permeable-crowned trees throughout crops to reduce both light and temperature, e.g. using Prosopis trees with millet crops in India, or partially shading taro in Hawaii.
Grass growth in temperate areas also increases with shelterbelt, but this may reflect the warmer conditions and lack of mechanical wind damage that such trees as tagasaste provide.
Trials of light-transmitting or thin crowned palms and legume trees would quickly show results, and there are a good many observations to suggest that (if water is sufficient) crops under leguminous trees do much better in the tropics than a crop standing on its own.
Part of the problem in tropics (both for biomass production and nutrition) is that non-adapted temperate crops are persistently grown there.
True tropical plants can not only stand much higher levels of light before saturation, but can also maintain photosynthesis at low (0.10%) carbon dioxide.
In summary, we do not have to accept the climatic factors of a site as unchangeable any more than we do its treelessness or state of soil erosion.
By sensible placement of our design components, we can create myriad small differences in local climatic effects on any site.
In the technical field, we can create useful conversions of energy from incoming energy fluxes such as wind and sun and produce energy for the site.
In the patterning of a site with trees, ponds, earth systems, or hedgerows, we can actively moderate for better climatic conditions, or to eliminate some local limiting factor.